Facilitating processing within computing environments supporting pageable guests

ABSTRACT

Processing within a computing environment that supports pageable guests is facilitated. Processing is facilitated in many ways, including, but not limited to, associating guest and host state information with guest blocks of storage; maintaining the state information in control blocks in host memory; enabling the changing of states; and using the state information in management decisions. In one particular example, the guest state includes an indication of usefulness and importance of memory contents to the guest, and the host state reflects the ease of access to memory contents. The host and guest state information is used in managing memory of the host and/or guests.

This application is a continuation of co-pending U.S. Ser. No. 14/296,686, entitled “FACILITATING PROCESSING WITHIN COMPUTING ENVIRONMENTS SUPPORTING PAGEABLE GUESTS,” filed Jun. 5, 2014, which is a continuation of U.S. Pat. No. 8,752,053, entitled “FACILITATING PROCESSING WITHIN COMPUTING ENVIRONMENTS SUPPORTING PAGEABLE GUESTS,” issued Jun. 10, 2014, which is a continuation of U.S. Pat. No. 8,387,049, entitled “FACILITATING PROCESSING WITHIN COMPUTING ENVIRONMENTS SUPPORTING PAGEABLE GUESTS,” issued Feb. 26, 2013, each of which is hereby incorporated herein by reference in its entirety.

BACKGROUND

This invention relates, in general, to computing environments that support pageable guests, and more particularly, to facilitating processing within such environments.

In computing environments that support pageable guests, processing is often complicated by multiple layers of resource management. One area of processing that has realized such complications is in the area of memory management. To manage memory in such an environment, it is common for both the pageable guests and their associated hosts to manage their respective memories causing redundancy that results in performance degradation.

As an example, in an environment in which a host implements hundreds to thousands of pageable guests, the host normally over-commits memory. Moreover, a paging operating system running in each guest may aggressively consume and also over-commit its memory. This over-commitment causes the guests' memory footprints to grow to such an extent that the host experiences excessively high paging rates. The overhead consumed by the host and guests managing their respective memories may result in severe guest performance degradation.

Thus, a need exists for a capability that facilitates processing within computing environments that support pageable guests. In one particular example, a need exists for a capability that facilitates more efficient memory management in those environments supporting pageable guests.

BRIEF SUMMARY

The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a computer program product for facilitating management of state information. The computer program product includes a computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method. The method includes associating guest state information of a guest to be executed by a virtual machine of a computing environment with a block of memory assigned to the guest, the associating comprising defining one or more available guest memory states, wherein a guest memory state describes a state of the block of memory as it relates to the guest, and wherein current guest state information is maintained in memory of a host of the computing environment, the host to manage the virtual machine, and wherein the current guest state information includes one of the available guest memory states indicating the current state of the block of memory assigned to the guest; and providing to the guest the guest state information of the block of memory assigned to the guest absent host involvement, wherein the guest state information provided to the guest comprises a particular guest memory state indicating a particular meaning contents of the block of memory have to the guest.

Methods and computer systems corresponding to the above-summarized system are also described and may be claimed herein.

Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

One or more aspects of the present invention are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1A depicts one embodiment of a computing environment to incorporate and use one or more aspects of the present invention;

FIG. 1B depicts further details of an input/output (I/O) subsystem of FIG. 1 a, in accordance with an aspect of the present invention;

FIG. 2 illustrates an association between an interpretative execution state description and a Collaborative Memory Management Backing Reclaim Log (CBRL), in accordance with an aspect of the present invention;

FIG. 3 depicts one example of a page status table entry (PGSTE), in accordance with an aspect of the present invention;

FIG. 4 depicts one example of a page table entry (PTE), in accordance with an aspect of the present invention;

FIG. 5 illustrates a correspondence of PTEs to PGSTEs, in accordance with an aspect of the present invention;

FIG. 6 depicts one example of the format of an Extract And Set Storage Attributes (ESSA) instruction, in accordance with an aspect of the present invention; and

FIG. 7 depicts one example of a finite state machine for the Extract and Set Storage Attributes instruction, in accordance with an aspect of the present invention.

DETAILED DESCRIPTION

In accordance with an aspect of the present invention, processing within computing environments supporting pageable guests is facilitated. Processing is facilitated in many ways, including, but not limited to, associating guest and host state information with guest blocks of memory or storage (storage and memory are used interchangeably herein); maintaining the state information in control blocks in host memory; enabling the changing of states by the guest; and using the state information in management decisions. In one particular example, the state information is used in managing memory of the host and/or guests.

One embodiment of a computing environment to incorporate and use one or more aspects of the present invention is described with reference to FIG. 1 a. Computing environment 100 is based, for instance, on the z/Architecture offered by International Business Machines Corporation, Armonk, N.Y. The z/Architecture is described in an IBM® publication entitled, “z/Architecture Principles of Operation,” IBM® Publication No. SA22-7832-02, June 2003, which is hereby incorporated herein by reference in its entirety. (IBM® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., USA. Other names used herein may be registered trademarks, trademarks, or product names of International Business Machines Corporation or other companies.) In one example, a computing environment based on the z/Architecture includes an eServer zSeries, offered by International Business Machines Corporation, Armonk, N.Y.

Computing environment 100 includes, for instance, a central processor complex (CPC) 102 providing virtual machine support. CPC 102 includes, for instance, one or more virtual machines 104, one or more central processors 106, at least one host 108 (e.g., a control program, such as a hypervisor), and an input/output subsystem 110, each of which is described below. The host and one or more virtual machines are executed by the central processors from a range of physical memory 114.

The virtual machine support of the CPC provides the ability to operate large numbers of virtual machines, each capable of hosting a guest operating system 112, such as Linux. Each virtual machine 104 is capable of functioning as a separate system. That is, each virtual machine can be independently reset, execute a guest operating system, and operate with different programs. An operating system or application program running in a virtual machine appears to have access to a full and complete system, but in reality, only a portion of the real system is available to the virtual machine.

In this particular example, the model of virtual machines is a V=V (i.e., pageable) model, in which the memory of a virtual machine is backed by host virtual memory, instead of real memory. Each virtual machine has a virtual linear memory space. The physical resources are owned by host 108, and the shared physical resources are assigned by the host to the guest operating systems, as needed, to meet their processing demands. This V=V virtual machine model assumes that the interactions between the guest operating systems and the physical shared machine resources are controlled by the host, since the large number of guests typically precludes the host from simply partitioning and assigning fixed hardware resources to the configured guests. Thus, for instance, the host pages containing recently referenced portions of virtual machine memory may be kept resident in physical memory, while less recently referenced portions are paged out to host auxiliary storage, allowing over-commitment of the aggregate memory requirements of virtual machines beyond the capacity of physical memory. One or more aspects of a V=V model are further described in an IBM® publication entitled “z/VM: Running Guest Operating Systems,” IBM Publication No. SC24-5997-02, October 2001, which is hereby incorporated herein by reference in its entirety.

Central processors 106 are physical processor resources that are assignable to a virtual machine. For instance, virtual machine 104 includes one or more virtual processors, each of which represents all or a share of a physical processor resource 106 that may be dynamically allocated to the virtual machine. Virtual machines 104 are managed by host 108. As examples, the host may be implemented in firmware running on processors 106 or be part of a host operating system executing on the machine. In one example, host 108 is a VM hypervisor, such as z/VM®, offered by International Business Machines Corporation, Armonk, N.Y. One embodiment of z/VM® is described in an IBM® publication entitled “z/VM: General Information Manual,” IBM® Publication No. GC24-5991-04, October 2001, which is hereby incorporated herein by reference in its entirety.

Input/output subsystem 110 directs the flow of information between devices and main storage. It is coupled to the central processing complex, in that it can be part of the central processing complex or separate therefrom. The I/O subsystem relieves the central processors of the task of communicating directly with the I/O devices coupled to the CPC and permits data processing to proceed concurrently with I/O processing. In one embodiment, I/O subsystem 110 includes a plurality of adapters 120 (FIG. 1 b) coupled to a plurality of I/O devices 122. A particular adapter may be coupled to one or more I/O devices and an I/O device may be coupled to one or more adapters.

In accordance with an aspect of the present invention, processing within computing environment 100 is facilitated. Many aspects of processing may be facilitated, but as one example, an embodiment is described herein that relates to facilitating memory management. Specifically, a Collaborative Memory Management Facility (CMM) is described herein. However, although CMM is described herein, one or more aspects of the present invention can relate to and/or benefit other areas of processing.

The Collaborative Memory Management Facility is a facility that provides a vehicle for communicating granular page state information between a pageable guest and its host. This sharing of information between the guest and host provides the following benefits, as examples:

-   -   The host can make more intelligent decisions when selecting page         frames to reclaim (steal).     -   Host page-write overhead is eliminated for blocks whose contents         the guest has indicated are discardable.     -   The guest memory footprint is reduced allowing for greater         memory over-commitment.     -   If the guest is defining and managing virtual memory for its         underlying processes, the guest can make more intelligent         decisions when selecting blocks to reclaim (steal). In one         example, the guest makes these decisions based on host state         information.     -   The guest can make more intelligent decisions when assigning         blocks. In one example, the guest makes these decisions based on         host state information.     -   Guest block-clearing overhead is eliminated for blocks the host         has indicated contain zeros (i.e., the host has already         cleared).

To enable CMM in an environment based on the z/Architecture, a state description 200 (FIG. 2) is employed. The state description includes virtual machine state, enabling direct execution of the virtual machine by a suitably designed processor. One example of such an interpretive execution capability is described in an IBM® publication entitled, “IBM System/370 Extended Architecture: Interpretive Execution,” IBM Publication No. SA22-7095 (1985), which is hereby incorporated herein by reference in its entirety.

State description 200 includes an enablement control bit (C) 202 for CMM. When this bit is one, the CMM facility is available to the guest and the guest may invoke a service (e.g., an Extract And Set Storage Attributes (ESSA) instruction) to interrogate and manipulate the block states associated with each guest block. In response to invoking the service, in one embodiment, a central processor interpretively executes the ESSA instruction via, for instance, the Interpretive Execution (SIE) architecture (a.k.a., the Start Interpretive Execution (SIE) architecture).

When the CMM enablement control bit is zero, the central processor does not interpretively execute the ESSA instruction. Thus, if a guest that is not enabled for CMM attempts to issue the ESSA instruction, an instruction interception occurs. This gives the host the option of simulating the ESSA instruction or presenting an operation exception program interruption to the guest.

In addition to control bit 202 used to enable CMM, state description 200 also includes a pointer 204 (CBRLO—CMM Backing Reclaim Log (CBRL) Origin) to a control block 206, referred to as the CMM backing reclaim log (CBRL). CBRL is auxiliary to the state description and includes a plurality of entries 208 (e.g., 511 8-byte entries). An offset to the next available entry in the CBRL is in the state description at 210 (NCEO—Next CBRL Entry Offset). Each CBRL entry that is at a location before the offset includes the guest absolute address of a guest block whose backing auxiliary storage can be reclaimed by the host.

The Collaborative Memory Management Facility of one or more aspects of the present invention includes, for instance, the following features, which are described in further detail herein:

-   -   The association of guest and host state information with each         guest block. The state information is maintained in control         blocks in host memory.     -   An Extract and Set Storage Attributes (ESSA) guest service that         allows the guest to directly determine the guest and host states         of a guest block without host involvement. The ESSA service also         allows the guest to optionally change the guest state of the         block. The ESSA service does this by directly accessing the host         control blocks containing the state information without host         involvement. In some cases, the changing of the guest state of a         block also results in the changing of the host state of the         block and possibly recording the block address in the CBRL.     -   Enhanced host control blocks for containing the host and guest         state information.     -   Enhancements to host and guest behaviors that are driven by         changes to the guest block states.

The association of guest and host state information with guest blocks includes the defining of available host states. As examples, the following host states are defined:

-   -   1. Resident (r) state: The guest block is present in a host page         frame.         -   A host page frame (a.k.a., frame, page frame) is a block             (e.g., 4K-byte) of host real memory that is used to contain,             or back, host pages that contain guest blocks (a.k.a.,             block). A host page or page is a page (e.g., 4K-byte) of             virtual memory that is used to implement a block of guest             memory. A block (a.k.a., guest block) is a block (e.g.,             4K-byte) of memory (on, for instance, a 4K-byte boundary)             that the guest views as a block of its physical (or             absolute) memory.     -   2. Preserved (p) state: The guest block is not present in a host         page frame, but has been preserved by the host in some auxiliary         storage.     -   3. Logically Zero (z) state: The guest block is not present in a         host page frame and the contents of the guest block are known to         be zeros.         -   The logically zero state is the initial (or default) host             state.

The association of guest and host state information also includes the defining of available guest states. As examples, the following guest states are defined:

-   -   1. Stable (S) state: The contents of a stable block remain equal         to what was set by the guest. The host is responsible for         preserving the contents of a block in the stable state, if the         backing page frame is reclaimed.         -   The stable state is the default guest state of a block. When             the guest changes the state of a block to a state other than             stable, the guest is to change the state back to the stable             state in order to ensure the contents are preserved by the             host.     -   2. Unused (U) state: The contents of an unused block are not         meaningful to the guest. After the guest sets the state of a         block to the unused state, the host may at any time discard the         contents of the block and reclaim the backing page frame. When         the host discards the contents of the block, it changes the host         state to z.         -   The guest is not to reference a block in the unused state;             otherwise, an addressing exception may occur.     -   3. Volatile (V) state: The contents of a volatile block are         meaningful to the guest, but the host may at any time discard         the contents of the block and reclaim the backing page frame.         The guest can tolerate such loss of the block contents because         it has the ability to recreate them. If the host reclaims the         backing page frame, the host changes the host state of the block         to z.         -   The guest may attempt to reference the contents of a block             in the guest volatile state. This will either succeed, if             the guest/host state of the block is Vr, or will result in a             block volatility exception, if the guest/host state of the             block is Vz.         -   Any changes the guest may make to the contents of a block in             the guest volatile state will be lost, if the block is             discarded.     -   4. Potentially Volatile (P) state: The contents of a potentially         volatile block are meaningful to the guest, but based upon guest         change history, the host either may discard or should preserve         the contents of the block.         -   If the change indicator associated with the block indicates             that the block has not been changed, the host may at any             time discard the contents of the block and reclaim the             backing page frame. The guest can tolerate such a loss of             the block contents, because it has the ability to recreate             them. If the host discards a potentially volatile block, the             host changes the guest/host state of the block to Vz.         -   If the change indicator associated with the block indicates             that the block has been changed, the host preserves the             contents of the block. When the host preserves the contents             on auxiliary storage, it changes the guest/host state of the             block from Pr to Sp.         -   The P state offers the benefits of both the V and S states.             This allows the guest to change the contents of blocks in             the P guest state, ensuring block content preservation by             the host. For those blocks in the P guest state that are not             changed by the guest, the host may efficiently discard the             contents and reclaim the host page frame without incurring             the overhead associated with block content preservation.

In accordance with an aspect of the present invention, the machine (e.g., firmware other than the guests and host) and the host ensure that the state of the guest block is in one of the following permissible guest/host block states: Sr, Sp, Sz, Ur, Uz, Vr, Vz, or Pr.

The state information for guest blocks is maintained, for instance, in host page tables (PTs) and page status tables (PGSTs) that describe a guest's memory. These tables include, for instance, one or more page table entries (PTEs) and one or more page status table entries (PGSTEs), respectively, which are described in further detail below.

One example of a page status table entry 300 is described with reference to FIG. 3. Page status table entry 300 includes, for instance, the following:

-   -   (a) Acc 302: Access control key;     -   (b) FP 304: Fetch protection indicator;     -   (c) Page Control Interlock (PCL) 306: This is the interlock         control for serializing updates to a page table entry (PTE) and         corresponding PGSTE, except for the PGSTE status area and PGSTE         bits that are marked as reserved.     -   (d) HR 308: Host reference backup indicator;     -   (e) HC 310: Host change backup indicator;     -   (f) GR 312: Guest reference backup indicator;     -   (g) GC 314: Guest change backup indicator;     -   (h) Status 316: Intended for host program use.     -   (i) Page Content Logically Zero Indicator (Z) 318: This bit is         meaningful when the corresponding PTE page invalid (PTE.I) bit         (described below) is one.         -   When Z is one, the content of the page that is described by             this PGSTE and corresponding PTE is considered to be zero.             Any prior content of the page does not have to be preserved             and may be replaced by a page of zeros.         -   When Z is zero, the content of the page described by the             PGSTE and corresponding PTE is not considered to be zero.             The content of the page is preserved by the host.         -   When the Z bit is one and the corresponding PTE.I bit is             one, the host state is z. This means that the page content             may be replaced by the host or by a function of the Host             Page Management Assist facility which is described in U.S.             Ser. No. 10/855,200 entitled “Interpreting I/O Operation             Requests From Pageable Guests Without Host Intervention,”             Easton et al., IBM Docket No. POU920030028US1, filed May 27,             2004, which is hereby incorporated herein by reference in             its entirety.         -   When the Z bit is one, the corresponding PTE.I bit is one,             and the page content is replaced, the page should be             replaced by associating it with a frame that has been set to             zeros.         -   When the Z bit is zero and the PTE invalid bit is one, the             host state is p.     -   (i) Page Class (PC) 320: When zero, the page described by the         PGSTE and corresponding PTE is a class 0 page and the delta         pinned page count array (DPPCA) for class 0 pages is used for         counting pinning and unpinning operations for the page. When         one, the page described by the PGSTE and corresponding PTE is a         class 1 page and the DPPCA for class 1 pages is used for         counting pinning and unpinning operations for the page.     -   (j) Pin Count Overflow (PCO) 322: When one, the pin count field         is in an overflow state. When zero, the pin count field is not         in an overflow state. In this case, the total pin count is kept         by the host in another data structure not accessed by the         machine.     -   (k) Frame Descriptor On Processed Frame Descriptor List (FPL)         324: When one, a frame descriptor for the page described by the         PGSTE and corresponding PTE is in a processed frame descriptor         list. The frame descriptor identifies the host page frame that         was used by the HPMA resolve host page function for the page.     -   (l) Page Content Replacement Requested (PCR) 326: When one, page         content replacement was requested when the HPMA resolve host         page function was invoked for the page represented by the PGSTE         and corresponding PTE.     -   (m) Usage State (US) 328: Indicates whether the guest state is         S, U, V or P.     -   (n) Status 330: Intended for host program use.     -   (o) Pin Count 332: An unsigned binary integer count used to         indicate whether the content of the host virtual page         represented by the PGSTE and corresponding PTE is pinned in the         real host page frame specified by the page frame real address         field of the PTE. When the value of this field is greater than         zero or the page count overflow (PCO) bit is one, the         corresponding page is considered to be pinned. When the value of         this field is zero and the PCO bit is zero, the corresponding         page is not considered to be pinned.         -   At the time a page is pinned by either the host or the CPU,             this field should be incremented by 1. At the time a page is             unpinned by either the host or the CPU, this field should be             decremented by 1.         -   When the value of the pin count field is greater than zero             or the PCO bit is one, the corresponding PTE.I (page             invalid) bit is to be zero. Otherwise, unpredictable results             may occur.         -   While a page is pinned, the host program should not change             the contents of the PTE page frame real address (PFRA)             field, the setting of the PTE page invalid (I) bit, or the             setting of the page protection (P) bit in the PTE or segment             table entry (STE). Otherwise unpredictable results may             occur.

One or more of the PGSTE fields described above are provided for completeness, but are not needed for one or more aspects of the present invention.

A PGSTE corresponds to a page table entry (PTE), an example of which is described with reference to FIG. 4. A page table entry 400 includes, for instance:

-   -   (a) Page Frame Real Address (PFRA) 402: This field provides the         leftmost bits of a real (in this case, host real) storage         address. When these bits are concatenated with the byte index         field of the virtual address on the right, the real address is         obtained.     -   (b) Page Invalid Indicator (I) 404: This field controls whether         the page associated with the page table entry is available. When         the indicator is zero, address translation proceeds by using the         page table entry. Further, the host state is r. When the         indicator is one, the page table entry cannot be used for         translation, and the host state is p or z, as determined by         PGSTE.Z.     -   (c) Page Protection Indicator 406: This field controls whether         store accesses are permitted into the page.

Further details regarding page table entries and page tables, as well as segment table entries mentioned herein, are provided in an IBM® publication entitled, “z/Architecture Principles of Operation,” IBM® Publication No. SA22-7832-02, June 2003, which is hereby incorporated herein by reference in its entirety. Moreover, further details regarding the PGSTE are described in U.S. Pat. No. 7,941,799 entitled “Interpreting I/O Operation Requests From Pageable Guests Without Host Intervention,” Easton et al., IBM Docket No. POU920030028US1, issued May 10, 2011, which is hereby incorporated herein by reference in its entirety.

In one embodiment, there is one page status table per page table, the page status table is the same size as the page table, a page status table entry is the same size as a page table entry, and the page status table is located at a fixed displacement (in host real memory) from the page table. Thus, there is a one-to-one correspondence between each page table entry and page status table entry. Given the host's virtual address of a page, both the machine and the host can easily locate the page status table entry that corresponds to a page table entry for a guest block. This is illustrated in FIG. 5, in which PTO is page table (PT) origin; I is the PTE invalid bit; Z is the PGSTE logically zero bit; and US is the PGSTE usage state field.

In order for a guest to extract the current guest and host block states from the PGSTE and to optionally set the guest state, a service is provided, in accordance with an aspect of the present invention. This service is referred to herein as the Extract and Set Storage Attributes (ESSA) service. This service can be implemented in many different ways including, but not limited to, as an instruction implemented in hardware or firmware, as a hypervisor service call, etc. In the embodiment described herein, it is implemented as an instruction, as one example, which is executed by the machine without intervention by the host, at the request of a guest.

The Extract And Set Storage Attributes instruction is valid for pageable guests for which the CMM facility is enabled. One example of a format of an ESSA instruction is described with reference to FIG. 6. An Extract And Set Storage Attributes instruction 600 includes an operation code 602 specifying that this is the ESSA instruction; an M₃ field 604 (input) indicating the operation to be performed; a designation 606 (R₁) for an output register into which the guest and host block states are extracted; and a designation 608 (R₂) for an input register which designates the guest absolute address of the block for which the block states are to be extracted and optionally set, per the operation code. With this instruction, the guest state and the host state of the block designated by the second operand are extracted into the first operand location. The guest state and host state may be optionally set based on the value of the M₃ field. Asynchronous to the execution of the instruction, either or both the guest state and the host state may be changed.

The M₃ field designates an operation request code specifying the operation to be performed. Example operations include:

-   -   Extract Block Attributes: The current guest and host states of         the designated block are extracted. No change is made to either         state.     -   In other embodiments, for any of the operations herein, the         guest or host state may be extracted without the other.     -   Set Stable State: The current guest and host states of the         designated block are extracted. Following extraction of the         states, the guest state is set to the stable state (S).     -   By extracting the host state, the guest is able to recognize         when the host state is logically zero, and bypass reclearing the         page if it is logically zero.     -   Set Unused State: The current guest and host states are         extracted. Following extraction, the guest state is set to the         unused state (U). Further, if the host block state is the         preserved state (p), the contents of the block are discarded,         the guest reference and change bits are set to zero, the address         of the block is recorded in the CBRL, and the host block state         is set to the logically zero state (z).     -   Recording an address in the CBRL includes, for instance, storing         the address in the next available entry, as designated by the         CBRLO and NCEO, and incrementing the NCEO by 8. However, if the         NCEO is already at its limit (for instance, 4088), indicating         that the CBRL is full, then instead, execution of the ESSA         instruction is suppressed, and an interception is presented to         the host, allowing the host to empty the CBRL.

Set Volatile State: The current guest and host block states are extracted. Following extraction, the guest block state is set to the volatile state (V). Also, if the host block state is the preserved state (p), the contents of the block are discarded, the guest reference and change bits are set to zero, the address of the block is recorded in the CBRL (as described above) and the host block state is set to the logically zero state (z).

Alternatively, if the host block state is resident (r), the contents are not immediately discarded, but the host may discard them subsequently, changing the host block state to the logically zero state (z).

Set Potentially Volatile State: The current guest and host block states are extracted. Following extraction, one of the following occurs:

-   -   (1) If the host block state is the resident state (r), the guest         block state is set to the potentially volatile state (P).     -   (2) If the host block state is the preserved state (p) and the         block has been changed by the guest, the guest block state         remains the stable state (S) (as shown in FIG. 7, described         below, the preserved host state only occurs in conjunction with         the stable guest state) and the host state remains the preserved         state.         -   (3) If the host block state is the preserved state (p) and             the block has not been changed by the guest, the contents of             the block are discarded, the guest reference and change bits             are set to zero, the address of the block is recorded in the             CBRL, the guest block state is set to the volatile state             (V), and the host block state is set to the logically zero             state (z).         -   (4) If the host block state is the logically zero state (z),             the guest block state is set to the volatile state (V).     -   Set Stable and Make Resident: The current guest and host block         states are extracted, and the guest state is set to the stable         state (S). Also, if the host block state is not in the resident         state, the following occurs: if a Host Page Management Assist         facility (HPMA) is installed and enabled, it is invoked to         attempt to make the block resident. If the Host Page Management         Assist facility is not installed, is not enabled or fails to         make the block resident, an instruction interception is         recognized, leaving the original guest and host states         unchanged. In this case, the host makes the block stable and         resident, which may involve restoring the block from auxiliary         storage. (Host Page Management Assist is described in detail in         U.S. Ser. No. 10/855,200 entitled “Interpreting I/O Operation         Requests From Pageable Guests Without Host Intervention,” Easton         et al., IBM Docket No. POU920030028US1, filed May 27, 2004,         which is hereby incorporated herein by reference in its         entirety.)     -   Set Stable If Resident: The current guest and host block states         are extracted. Following extraction, the guest state is set to         the stable state (S), if the host state is the resident state         (r).

In one embodiment, the set operations accomplish the extracting and setting in an atomic operation. In an alternate embodiment, the setting may be performed without extracting or by extracting only the guest or host state.

As described above, if the program issues an Extract And Set Storage Attributes instruction which would result in an impermissible combination of guest and host states, the machine will replace the impermissible combination with a permissible combination. The table below summarizes which combinations are permissible and which are not. The table also shows the state combinations (in parentheses) which replace the impermissible combinations.

Host States r p z Guest S Y Y Y States U Y N(Uz)¹ Y V Y N(Vz)² Y P Y N(Sp)³ (Vz)³ N(Vz)⁴ Legend: r—Resident host state p—Preserved host state z—Logically-zero content host state S—Stable guest state U—Unused guest state V—Volatile guest state P—Potentially volatile guest state Y—Yes - permissible N—No - not permissible The footnotes on the guest/host state combinations shown in parentheses are described below: ¹Use of ESSA instruction to set the guest state of a block in the preserved state to the unused state will result in discarding the block contents by changing the guest state to the unused state and the host state to the logically zero state. ²Use of the ESSA instruction to set the guest state of a block in the preserved state to the volatile state will result in discarding the block contents by changing the guest state to the volatile state and the host state to the logically zero state. ³Use of the ESSA instruction to set the guest state of a block in the preserved state to the potentially volatile state will result in the guest state remaining the stable state and the host state remaining the preserved state, if the block has been changed, or will result in discarding the block by changing the guest state to the volatile state and the host state to the logically zero state, if the block has not been changed. ⁴Use of the ESSA instruction to set the guest state of a block in the logically zero host state to the potentially volatile state will result in changing the guest state to the volatile state and the host state will remain the logically zero state.

A state diagram representing transitions between the various states is depicted in FIG. 7. In one example, a finite state machine is built to implement the states and the transitions between the states. The states illustrated in the state diagram are described above. For example, Ur indicates an unused guest state/resident host state; Uz indicates an unused guest state/logically zero host state; Sr—stable guest state/resident host state; Sp—stable guest state/preserved host state; Sz—stable guest state/logically zero host state. The Sz state is the initial (default) guest/host state for a page. Since Sz is the initial state, a virtual machine that does not invoke the ESSA service has its storage managed, as if all the guest blocks are in the stable state; Vr—volatile guest state/resident host state; Vz—volatile guest state/logically zero host state; Pr—potentially volatile guest state/resident host state.

Transitions depicted in the figure include host-initiated operations, such as stealing a frame backing a resident page, paging into or out of auxiliary storage; guest-initiated operations through the ESSA service or through references to memory locations; and implicit operations, such as discarding the page contents or backing storage, which arise from the explicit host and guest operations. Further, in the figure, a block is considered “dirty” (guest page changed), if the guest has no copy of the content on backing storage. Likewise, a block is clean (guest page unchanged), if the guest has a copy of the content on backing storage. Resolve stands for backing a block indicated as Sz with a real resident zero filled memory block.

When the ESSA instruction completes, the general register designated by the R₁ field contains the guest state and host state of the designated block before any specified state change is made. As one example, this register includes guest state (a.k.a., block usage state (US)) and host state (a.k.a., block content state (CS)). The guest state includes a value indicating the block usage state of the designated block including, for instance, stable state, unused state, potentially volatile state, and volatile state. The host state includes a value indicating the block content state of the designated block including, for instance, resident state, preserved state, and logically zero state.

When the ESSA instruction recognizes, by analysis of the guest and host states and requested operation, that the contents of a block in backing auxiliary storage may be discarded and the CBRL for the guest is not full, the host state of the block is set to z, and an entry is added to the CBRL that includes the guest address of the block. Later, when the processor exits from interpretive execution of the guest, the host processes the CBRL and reclaims the backing page frames and associated backing auxiliary storage of the guest blocks that are recorded in the CBRL. After this processing, the CBRL is empty of entries (i.e., the host sets the NCEO filed in the state description to zero).

When the ESSA instruction recognizes that the contents of a block in backing auxiliary storage may be discarded and the CBRL is full, an instruction interception occurs, the host processes the CBRL, as described above, either simulates the ESSA instruction, or adjusts guest state so that the machine will re-execute it, and then redispatches the guest.

At any time, the host may reclaim the frame for a block that is in the S guest state or for a block that has been changed and is in the P guest state. In these cases, the host preserves the page in auxiliary storage and changes the host state to p, and the guest state to S, if not already so.

The host may also reclaim the frame for a block that is in the U or V guest state or for a block that has not been changed and is in the P guest state. In these cases, the host does not preserve the block contents, but rather places the page into the z host state and, if it was in the P guest state, changes the guest state to V. For maximum storage management efficiency, the host should reclaim frames for blocks that are in the U guest state before reclaiming frames for blocks that are in the S guest state. Similarly, there may be value in reclaiming V or unchanged P frames in preference to S frames.

In summary, the Extract And Set Storage Attributes service locates the host PTE and PGSTE for the designated guest block (specified as guest absolute address); obtains the page control interlock (as is done for guest storage key operations and HPMA operations), issuing an instruction interception, if the interlock is already held; fetches current page attributes: bits from PGSTE.US field, plus PTE.I; optionally sets attributes in the PGSTE; for Set Stable and Make Resident operations, invokes the HPMA resolve host page function to make a page resident and clears PTE.I (e.g., sets PTE.I to zero); immediately discards page contents for Set Unused, Set Volatile, and Set Potentially Volatile states, if the host state is preserved—host auxiliary storage reclamation deferred via CMM backing reclaim log (CBRL); releases page control interlock; and returns old page attributes in the output register. This service is invoked by a guest, when, for instance, the guest wishes to interrogate or change the state of a block of memory used by the guest.

The host is able to access the guest states in PGSTE and make memory management decisions based on those states. For instance, it can determine which pages are to be reclaimed first and whether the backing storage needs to be preserved. This processing occurs asynchronously to execution of ESSA. As a result of this processing, guests states may be changed by the host.

With CMM, various exceptions may be recognized. Examples of these exceptions include a block volatility exception, which is recognized for a guest when the guest references a block that is in the guest/host state of Vz; and an addressing exception, which is recognized when a guest references a block in the Uz state. Blocks in the Vz or Uz state are treated as if they are not part of the guest configuration by non-CPU entities (e.g., an I/O channel subsystem), resulting in exception conditions appropriate for those entities.

In one embodiment, in response to the guest receiving a block volatility exception (or other notification), the guest recreates the content of the discarded block. The content may be recreated into the same block or an alternate block. Recreation may be performed by, for instance, reading the contents from a storage medium via guest input/output (I/O) operations, or by other techniques.

As one example, to recreate the content into an alternate block, a block is selected and the content is written into the selected block (e.g., via an I/O operation or other operation). Then, the selected block is switched with the discarded block.

To perform the switch, a service is provided that is used by the guest to swap the host translations (or mappings) of the two blocks. This service may be implemented in many ways, including but not limited to, as an instruction implemented in hardware or firmware, or as a hypervisor service call, etc. The service atomically replaces the contents of the PTE and host and guest state information in the PGSTE (collectively, referred to herein as translation and state information) of the discarded block with translation and state information (e.g., the contents of the PTE and host and guest state information in the PGSTE) of the recreated block, and vice versa.

In a further embodiment, the contents of the PTE and host and guest state information of the recreated block replace the corresponding contents in the discarded block without having the contents of the discarded block replace the contents of the recreated block. Alternatively, the recreated block could, for instance, be set to the Uz state.

Described in detail above is a capability for facilitating processing in a computing environment that supports pageable guests. One particular area of processing that is facilitated is in the area of memory management. For instance, a Collaborative Memory Management facility is provided that enables collaboration among a host, the machine and its pageable guests in managing their respective memories. It includes communicating block state information between the guest and the host, and based on that state information, the host and guest taking certain actions to more efficiently manage memory. Advantageously, in one example, the solution provided enables memory footprints and paging rates associated with the execution of n=many virtual servers to be reduced, thereby providing corresponding guest and host performance improvements.

With one or more aspects of the present invention, second level virtual storage is more efficiently implemented and managed. As an example, page state information may be associated with guest blocks used to back the second level virtual storage and manipulated and interrogated by both the host and guest in order that each can more efficiently manage its storage. Dynamic collaboration between host and guest with respect to paging information, specifically page state information, is provided.

In virtual environments, the host (e.g., hypervisor) is to faithfully emulate the underlying architecture. Therefore, previously, irrespective of the content of a page in the guest, the host would back that page up. That is, the host assumed (sometimes incorrectly) that the contents of all guest pages were needed by the guest. However, in accordance with an aspect of the present invention, by the guest providing the host with certain information about the guest state and its ability to regenerate content, if necessary, the host can circumvent certain operations reducing overhead and latency on memory page operations.

Advantageously, various benefits are realized from one or more aspects of the present invention. These benefits include, for instance, host memory management efficiency, in which there is more intelligent selection of page frames to be reclaimed (for instance, by reclaiming frames backing unused pages in preference to those backing other pages) and reduced reclaim overhead (avoiding page writes where possible); and guest memory management efficiency, in which double clearing of a page on reuse is avoided (by recognizing that the page has been freshly instantiated from the logically zero state); and more intelligent decisions are made in assigning and/or reclaiming blocks (by favoring reuse of already-resident blocks over use of blocks not currently resident). Additionally, the guest memory footprint is reduced at lesser impact to the guest (by trimming unused blocks), allowing for greater memory over-commit ratios.

While various examples and embodiments are described herein, these are only examples. Many variations to these examples are included within the scope of the present invention. For example, the computing environment described herein is only one example. Many other environments may include one or more aspects of the present invention. For instance, different types of processors, guests and/or hosts may be employed. Moreover, other types of architectures can employ one or more aspects of the present invention.

Although the present invention has been described in the context of host and guest operating systems, these techniques could also be applied for collaboration between a single operating system and a sophisticated application which manages its own memory pool, such as a buffer pool for database or networking software. Many other variations are also possible.

Further, in the examples of the data structures described herein, there may be many variations, including, but not limited to a different number of bits; bits in a different order; more, fewer or different bits than described therein; more, fewer or different fields; fields in a differing order; different sizes of fields; etc. Again, these fields are only provided as an example, and many variations may be included. Further, indicators and/or controls described herein may be of many different forms. For instance, they may be represented in a manner other than by bits. As another example, guest state information may include a more granular indication of the degree of importance of the block contents to the guest, as a further guide to host page selection decisions.

Yet further, the guest and/or host state information may be maintained or provided by control blocks other than the PGSTE and PTE.

As used herein, the term “page” is used to refer to a fixed size or a predefined size area of storage. The size of the page can vary, although in the examples provided herein, a page is 4K. Similarly, a storage block is a block of storage and as used herein, is equivalent to a page of storage. However, in other embodiments, there may be different sizes of blocks of storage and/or pages. Many other alternatives are possible. Further, although terms such as “tables”, etc. are used herein, any types of data structures may be used. Again, those mentioned herein are just examples.

The capabilities of one or more aspects of the present invention can be implemented in software, firmware, hardware or some combination thereof.

One or more aspects of the present invention can be included in an article of manufacture (e.g., one or more computer program products) having, for instance, computer usable media. The media has therein, for instance, computer readable program code means or logic (e.g., instructions, code, commands, etc.) to provide and facilitate the capabilities of the present invention. The article of manufacture can be included as a part of a computer system or sold separately.

Additionally, at least one program storage device readable by a machine embodying at least one program of instructions executable by the machine to perform the capabilities of the present invention can be provided.

The flow diagrams depicted herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.

Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims. 

What is claimed is:
 1. A computer program product for facilitating management of state information, said computer program product comprising: a computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: associating guest state information of a guest to be executed by a virtual machine of a computing environment with a block of memory assigned to the guest, the associating comprising defining one or more available guest memory states, wherein a guest memory state describes a state of the block of memory as it relates to the guest, and wherein current guest state information is maintained in memory of a host of the computing environment, the host to manage the virtual machine, and wherein the current guest state information comprises one of the available guest memory states indicating the current state of the block of memory assigned to the guest; and providing to the guest the guest state information of the block of memory assigned to the guest absent host involvement, wherein the guest state information provided to the guest comprises a particular guest memory state indicating a particular meaning contents of the block of memory have to the guest.
 2. The computer program product of claim 1, wherein the current guest state information is maintained in one or more control blocks in memory of the host, and wherein the providing comprises directly accessing the one or more control blocks without host involvement to provide the guest state information to the guest.
 3. The computer program product of claim 2, wherein the method further comprises changing, by the guest, guest state information maintained in at least one control block of the one or more control blocks maintained in memory of the host, wherein the changing is performed via an instruction issued by the guest and absent host intervention.
 4. The computer program product of claim 1, wherein the virtual machine is of a pageable model in which memory of the virtual machine is backed by host virtual memory, and wherein the method further comprises reclaiming by the host one or more backing frames of memory, wherein the reclaiming results in changing guest state information maintained in memory of the host, wherein the host selects one or more backing frames to reclaim based in part on the guest state information.
 5. The computer program product of claim 1, wherein the particular guest memory state comprises a volatile state in which contents of the block of memory are meaningful to the guest, but can be discarded by the host, with notification to the guest, since the guest can recreate the contents.
 6. The computer program product of claim 5, in which the notification is by an interruption at the time of an attempted reference to the discarded contents by the guest.
 7. The computer program product of claim 5, wherein the method further comprises recreating by the guest the content of the discarded block, based on the guest receiving the notification.
 8. The computer program product of claim 7, wherein the recreating comprises recreating the content of the discarded block in an alternate block.
 9. The computer program product of claim 8, wherein the method further comprises switching the discarded block with the alternate block, wherein the switching comprises replacing translation and state information of the discarded block with translation and state information of the alternate block.
 10. The computer program product of claim 1, wherein the particular guest memory state comprises a potentially volatile state in which contents of the block of memory are meaningful to the guest, but can be discarded by the host, based on contents being unchanged, since the guest can recreate the unchanged contents.
 11. A computer-implemented method of facilitating management of state information, said computer-implemented method comprising: associating guest state information of a guest to be executed by a virtual machine of a computing environment with a block of memory assigned to the guest, the associating comprising defining one or more available guest memory states, wherein a guest memory state describes a state of the block of memory as it relates to the guest, and wherein current guest state information is maintained in memory of a host of the computing environment, the host to manage the virtual machine, and wherein the current guest state information comprises one of the available guest memory states indicating the current state of the block of memory assigned to the guest; and providing to the guest the guest state information of the block of memory assigned to the guest absent host involvement, wherein the guest state information provided to the guest comprises a particular guest memory state indicating a particular meaning contents of the block of memory have to the guest.
 12. The computer-implemented method of claim 11, wherein the current guest state information is maintained in one or more control blocks in memory of the host, and wherein the providing comprises directly accessing the one or more control blocks without host involvement to provide the guest state information to the guest.
 13. The computer-implemented method of claim 11, wherein the virtual machine is of a pageable model in which memory of the virtual machine is backed by host virtual memory, and wherein the method further comprises reclaiming by the host one or more backing frames of memory, wherein the reclaiming results in changing guest state information maintained in memory of the host, wherein the host selects one or more backing frames to reclaim based in part on the guest state information.
 14. The computer-implemented method of claim 11, wherein the particular guest memory state comprises a volatile state in which contents of the block of memory are meaningful to the guest, but can be discarded by the host, with notification to the guest, since the guest can recreate the contents.
 15. The computer-implemented method of claim 11, wherein the particular guest memory state comprises a potentially volatile state in which contents of the block of memory are meaningful to the guest, but can be discarded by the host, based on contents being unchanged, since the guest can recreate the unchanged contents.
 16. A computer system for facilitating management of state information, said computer system comprising: a memory; and a processor in communication with the memory, wherein the computer system is configured to perform a method, said method comprising: associating guest state information of a guest to be executed by a virtual machine of a computing environment with a block of memory assigned to the guest, the associating comprising defining one or more available guest memory states, wherein a guest memory state describes a state of the block of memory as it relates to the guest, and wherein current guest state information is maintained in memory of a host of the computing environment, the host to manage the virtual machine, and wherein the current guest state information comprises one of the available guest memory states indicating the current state of the block of memory assigned to the guest; and providing to the guest the guest state information of the block of memory assigned to the guest absent host involvement, wherein the guest state information provided to the guest comprises a particular guest memory state indicating a particular meaning contents of the block of memory have to the guest.
 17. The computer system of claim 16, wherein the current guest state information is maintained in one or more control blocks in memory of the host, and wherein the providing comprises directly accessing the one or more control blocks without host involvement to provide the guest state information to the guest.
 18. The computer system of claim 16, wherein the virtual machine is of a pageable model in which memory of the virtual machine is backed by host virtual memory, and wherein the method further comprises reclaiming by the host one or more backing frames of memory, wherein the reclaiming results in changing guest state information maintained in memory of the host, wherein the host selects one or more backing frames to reclaim based in part on the guest state information.
 19. The computer system of claim 16, wherein the particular guest memory state comprises a volatile state in which contents of the block of memory are meaningful to the guest, but can be discarded by the host, with notification to the guest, since the guest can recreate the contents.
 20. The computer system of claim 16, wherein the particular guest memory state comprises a potentially volatile state in which contents of the block of memory are meaningful to the guest, but can be discarded by the host, based on contents being unchanged, since the guest can recreate the unchanged contents. 